The process for the preparation of ethylene, a starting material with diverse uses in chemical syntheses, are presently almost exclusively based on the cracking of petroleum distillates or natural gas condensates for "wet" gas (ethane and higher hydrocarbons). Elaborate purification steps and gas separation procedures must still follow the industrial cracking process in order to obtain the ethylene in a purity necessary for the further processing. Less elaborate processing steps are necessary for the preparation of ethylene from ethane, but the availability of ethane is limited.
In contrast, methane is a raw material abundantly available in natural deposits. Natural gas contains up to over 90% methane. Therefore, there is interest in developing an economic process for the preparation of ethylene from methane by oxidative coupling.
Many reports have been published on the production of lower olefins from simple starting compounds. Thus, in Hydrocarbon Proc., Nov. 1982, p. 117, T. Inui et al. describe a process in which a mixture of lower olefins is obtained from methanol. In Hydrocarbon Proc., May 1983, p. 88, Y.C. Hu describes a process in which the synthesis gas is converted by the Fischer-Tropsch process into a mixture of olefins, paraffins and carbon dioxide. These processes have the particular disadvantage that the operations must be carried out under pressure and coke formation must be ascertained.
The conversion of methane in the presence of oxygen at atmospheric pressure in one reaction step has already been proposed in the past. Thus, in J. Catal., 73 (1982), pp. 9-19, G. E. Keller and M. M. Bhasin report a process in which the conversion was investigated in the presence of numerous metal oxides as catalyst at 500.degree. to 1000.degree. and a mixture of ethylene and ethane was formed as reaction products in addition to carbon monoxide and carbon dioxide. However, the disadvantage of a very low .sup.C O.sub.2 hydrocarbon selectivity is inherent in this process. To improve selectivity, these authors propose a method with elaborate and costly procedures requiring a high technological outlay.
With reference to the deficiencies of this publication, a process is now proposed in Unexamined West German Patent Application No. 32 37 079, which results in comparable, in part even better, selectivities and higher space-time yields of .sup.C O.sub.2 hydrocarbons even without costly and complex procedures (cf. Unexamined West German Patent Application No. 32 37 079, p. 2, last paragraph, to p. 3, paragraph 3). According to this publication, a process is described for the production of ethane and/or ethylene in which methane and oxygen are reacted in the presence of a catalyst in a fluidized bed or fixed in a reactor at temperatures between 500.degree. and 900.degree. C. within a certain range of partial pressure ratios of methane and oxygen. The catalyst is an oxide of polyvalent metals (cf. Unexamined West German Patent Application No. 32 37 079, p. 3, last paragraph). Cited as preferred catalysts are products with oxides of lead, antimony, tin, bismuth, cadmium, thallium and indium or mixtures thereof as active constituents, lead oxide or a mixture thereof with antimony oxide being especially preferred. The metal oxides can be used as such or dispersed on the surface of a carrier such as aluminum oxide or silicon dioxide (cf. claim 6 and p. 4, paragraph 4). However, as apparent from the results of specific examples in Table 1, even with the particularly preferred lead oxide (on a carrier), a selectivity for hydrocarbons of a maximum of only about 52.9% (7% methane conversion) is achieved. In 1984, at the 8th International Congress on Catalysts in Berlin, W. Hinsen, W. Bytyn, and M. Baerns reported on more extensive research to increase the selectivity. According to the proceedings of the "8th International Congress on Catalysts," Vol. III, pp. 581-592, Verlag Chemie, Weinheim, 1984, the hydrocarbon selectivity of the lead oxide catalyst can be raised by appropriate selection of the carrier and by the addition of alkali. Gamma-aluminum oxide proved the most suitable among the tested carriers: alpha-aluminum oxide, gamma-aluminum oxide, titanium dioxide, aluminum silicate and silica gel; it resulted in maximum selectivity of up to 57.7% (7.1% methane conversion). However, for a commercial exploitation of the process of the oxidative coupling of methane, the need exists for a substantial increase of the selectivity and of the methane conversion.
Therefore, the present invention has as its object the conversion, particularly into C.sub.2 hydrocarbons, of methane according to the process of oxidative coupling--whose fundamental principle is known--in a catalyzed reaction with higher selectivities and efficient conversions of methane.